1. Field of the Invention
This invention pertains to the field of electrochemistry. With more particularity, this invention pertains to thermally activated electrochemical cells. With greatest particularity, this invention pertains to thermally activated electrochemical cells having a composite anode.
2. Description of the Prior Art
Thermally activated electrochemical cells arranged as batteries have been used quite extensively in military applications. Commonly such a battery is used in missiles as a power source. Thermal batteries are selected for this purpose because of their long shelf life and compactness and ability to withstand shock and vibration. Typically these batteries include an electrolyte which under normal storage conditions, is solid and does not conduct electricity. When the battery and/or the electrolyte is heated to a predetermined temperature, as by a built-in pyrotechnic heat source, the electrolyte, upon changing to a molten state, ionically connects the electrodes of the battery to provide the desired electromotive force. Thermal batteries employed in the past and in some present applications in missile systems make use of a lithium chloride-potassium chloride (LiCl-KCl) mixture as the electrolyte, calcium metal as the anode and calcium chromate (CaCrO.sub.4) as the cathodic material. The relatively high melting point of the electrolyte limits the activation of the battery to temperatures above 352.degree. C., and thermal batteries using LiCl-KCl mixtures are generally designed to operate at internal temperatures of between 475.degree. and 550.degree. C.
Nitrate salts have been proposed for use in thermal batteries because of their low melting points. For example, see U.S. Pat. No. 4,260,667 hereby incorporated by reference. Potassium nitrate-lithium nitrate (KNO.sub.3 -LiNO.sub.3) mixtures melt at temperatures as low as 123.degree. C. The use of a lower melting point electrolyte can shorten the activation time required for a thermal battery and also reduce the weight of the heat source and insulation required to activate the battery. Also, nitrate salts are low-hazard materials, unlike chromates, which are recognized as health hazards (CaCrO.sub.4 has been confirmed as a carcinogen). The high rate discharge of prior nitrate salt-containing battery cells, however, has been limited by both the anode and the cathode.
Composite anodes for use in thermal batteries have been proposed utilizing lithium and a particulate metal such as iron. For example, see U.S. Pat. No. 4,221,849 hereby incorporated by reference. However, the composite lithium-iron anode material prepared as described in U.S. Pat. No. 4,221,849 has generally been used with lithium chloride and potassium chloride as the electrolyte. This electrolyte system does not melt until 352.degree. C. Also, the lithium-iron composite material displays a marked tendency to deflagrate on contact with nitrate melts over 180.degree. C. if the anode lithium content is greater than 10 percent by weight lithium.
The use of silver nitrate (AgNO.sub.3) is known to improve cathode performance. For example, see U.S. Pat. No. 4,416,958 hereby incorporated by reference.
Calcium and a lithium-boron alloy have both been previously used as the anode in molten nitrate salt experimental cells. Calcium anodes have suffered from anodic passivation, while having maximum current densities of around 100 mA/cm.sup.2 and maximum potential of -2.8 volts. Lithium-boron anodes gave anode potentials of about -3.0 volts at current densities of over 300 mA/cm.sup.2 with a usable temperature range of over 150.degree. C.